CSS 라이브 데모: text-decoration-thickness 데모 버튼을 클릭해 보세요!
selector {
    text-decoration-thickness: auto | from-font | <length-percentage>
}
/* 단일 키워드 값 */
text-decoration-thickness: auto;
text-decoration-thickness: from-font;

/* 길이 값 */
text-decoration-thickness: 0.12em;
text-decoration-thickness: 5px;

/* 백분율 값 */
text-decoration-thickness: 12%;

/* 전역 값 */
text-decoration-thickness: inherit;
text-decoration-thickness: initial;
text-decoration-thickness: revert;
text-decoration-thickness: revert-layer;
text-decoration-thickness: unset;
<p>텍스트 데코레이션 두께가 5px입니다.</p>
p {
    text-decoration-line: underline;
    text-decoration-color: red;
    text-decoration-style: solid;
    text-decoration-thickness: 5px;
}
실제 적용 모습